There have been many known attempts to produce a digital medical X-ray imager. The energy distribution of the X-rays in the image plane is determined by the irradiated subject matter, human tissue and bone, resulting in an almost continuous energy spectrum both spatially and on an intensity scale. The image is generally a low contrast image whose gray tones need to be preserved for accurate diagnosis. Such an X-ray image has conventionally be recorded successfully on film in an analogue way. Digital recording devices determine the intensity averaged over picture elements or pixels. The spacing of the pixels determines the resolution of the device. The intensity at each pixel is normally binned into one of a finite number of levels. Hence, the gray scale resolution is limited to this number of levels rather than being a continuous spectrum of values. All known devices attempt to optimize resolution and gray scale performance but are usually limited in one aspect or another.
The X-rays may be detected directly or indirectly, e.g. a direct conversion of X-rays to an electrical output or first a conversion to visible light and then recording the visible light pattern as shown schematically in FIG. 1.
Direct conversion devices include the use of an layer of selenium for attenuating the X-rays and generating free electron-hole pairs for collection by suitable electrodes.
Indirect conversion devices use phosphors for converting the X-rays into visible light. One improvement has been photostimulable phosphors, known as storage phosphors. These store the image for later activation by red light. The phosphor can be "read" by a scanning laser and emits blue light in accordance with the stored image. The emitted light is collected and detected by a photomultiplier.
Another improvement is represented by the XRII described in the article by P. M. DeGroot, "Image intensifier design and specifications", Proc. Summer School on Specification, Acceptance, Testing and Quality Control of Diagnostic X-ray Imaging Equipment, Woodbury, U.S.A., 1994, pages 429-60. In this device X-rays are converted to light in a large curved phosphor screen. The resulting fluorescence illuminates a photocathode which liberates electrons and is evaporated directly onto the inside of the phosphor. The electrons are accelerated through a large electric potential, e.g. 25 kV, and electrostatically focussed by electrodes onto a small diameter output phosphor which may be observed by a video camera. The device has the advantage that the electron energy is increased by the acceleration thus counteracting to a certain extent some of the conversion loss but has the serious disadvantages of being very bulky giving limited access to the patient, image distortion, loss of image contrast and high cost and complexity.
A microgap sensor is known from the article "The Micro-gap Chamber", by F. Angellini et alia, Nuclear Instruments & Methods in Physics and Research, Sect. A (1993), pages 69-77. Such devices can detect X-rays however they are mainly used for detection of particles and the resolution of the devices is poor. Microdot detectors as described by Boagi et. al. in the article "Further experimental results of gas microdot detectors", 4th International Conference on Position Sensitive Detectors, Manchester, Sep. 9-13, 1996 use an electrode arrangement as shown schematically in FIGS. 2A and B. FIG. 2A shows a top view of the electrodes which include anodes 6 in the center connected to an anode readout bus 4 and cathodes 7 arranged around each anode 6 and connected to a cathode readout bus. Between the cathode 7 and the anode 6 one or more field electrodes 5 may be placed. The planar electrode arrangement and read-out busses may be conveniently made using standard semiconductor processing techniques starting from a planar substrate such as a glass plate or a semiconductor, e.g. silicon wafer 9, oxidizing the wafer to form an insulating layer 8 and depositing the electrodes 5,6,7. A gap 3 (typically 3 mm) is provided between the planar electrode arrangement and a cathodic drift electrode 2 which is filled by an ionisable gas. An electric potential is provided between the drift electrode and the electrode arrangement. A high energy particle entering the device via the drift electrode 2 causes release of at least one electron. This electron is accelerated towards the nearest anode or anodes 6 causing further collisions with atoms of the ionisable gas thus releasing more electrons until an avalanche is produced. The arrival of the avalanche at one or more anodes 6 is detected by electronic circuitry connected to the read-out busses. These known devices are suitable for detecting high-energy particles but are unsuitable for recording a complex 2-D X-ray image.
An object of the present invention is to provide a particle detector or a radiation, especially an X-ray imager which provides good resolution and a wide intensity scale.
A further object of the present invention is to provide a particle detector or a radiation, especially an X-ray imager with reduced bulk.
Another object of the present invention is to provide a particle detector or a radiation, especially an X-ray imager with reduced manufacturing cost and complexity.
Yet another object of the present invention is to provide a large size particle detector or a radiation, especially an X-ray imager.